Internalization of insulin and its receptor have been examined in rat adipose cells at 37 C. Insulin and its receptor are rapidly translocated into the cell's high- and low-density microsomes; the receptor's vectorial disposition in the plasma membrane is inverted during internalization. Acute phosphorylation of the insulin receptor is not essential for internalization or the response to insulin. Insulin stimulates glucose transport in rat adipose cells through the translocation of glucose transporters from an intracellular pool to the plasma membrane. The intracellular pool of glucose transporters comprises as specialized membrane species and intracellular glucose transporters are processed during their cycling. Insulin appears to stimulate glucose transport specifically at a step in which glucose transporters in the plasma membrane become functional. A photoaffinity crosslinking agent has now been used to attach [3H] cytochalosin B to the glucose transportec. chronic hyperinsulinemia increases insulin binding and the capacity of rat adipose cells to transport and metabolize glucose; hyperinsulinemia enhances insulin's stimulatory action on glucose transport by increasing the intracellular pool of glucose transporters. Insulin resistant glucose transport induced by fasting is due to a depletion of intracellular glucose transporters; however, the hyperresponsive insulin-stimulated glucose transport activity associated with refeeding is not totally accounted for by a change in the number of glucose transporters. Catecholamines counterregulate insulin-stimulated glucose transport, but only in part by modulating the translocation of glucose transporters; this counterregulation appears to comprise both cAMP-independent and -dependent mechanisms. Insulin also induces and increase in IGF-II binding in isolated rat adipocytes by causing a steady state redistribution of receptors from a large intracellular pool to the plasma membrane.